The present invention relates to zinc powders coated with bismuth which are advantageous as negative electrode materials for use in alkaline cells and a method of preparing the same.
Zinc has favorably been used as a negative electrode material for use in a cell since it has an appropriate potential when used as a negative electrode in an aqueous solution system, as well as it is low-priced. However, its corrosion potential is more base than the decomposition potential of water so that, when it contacts an electrolyte, it evolves a hydrogen gas. Therefore, a method has conventionally been dominantly performed that suppresses a gas evolution amount by raising hydrogen overvoltage through usage of amalgamated zinc which is prepared by adding mercury to zinc.
However, the mercury contained in the cell has caused an environmental pollution problem. Recent studies of zinc powders to be used as a negative electrode have been directed to the development of a mercury-free zinc alloy which can substitute for amalgamated zinc and to the suppression of hydrogen gas evolution by the use of such mercury-free zinc alloy. In connection with this development, various attempts have been made inclusive of the selection of an optimal zinc alloy composition, the surface treatment of zinc particles, the selection of an optimal electrolyte composition, the addition of a corrosion inhibitor to the electrolyte or the like. Particularly, a zinc alloy containing one or more elemental components selected from the group consisting of Al, Bi, In, Ga, Sn, Pb and the like was employed as a negative electrode material to provide a mercury-free alkaline cell in which the compositional ratio of these alloying elements was rendered optimal in suppressing the hydrogen gas evolution (see Japanese Unexamined Patent Publications (Kokai) No. 299082/1993, No. 40926/1998 and Japanese Examined Patent Publication (Kokoku) No. 54705/1995).
However, from the standpoint of safety in terms of preventing liquid leakage or burst of the cell to be caused by the hydrogen gas evolution, the development of a zinc powder for use as a negative electrode material capable of further suppressing the hydrogen gas evolution is a principal problem basically inherent in a cell which employs an alkaline electrolyte. Particularly, since the hydrogen gas evolution has not been sufficiently suppressed in a button-typed cell which uses zinc powders having a small average particle size and large relative surface areas, it is a current situation that a few % by weight of amalgamated zinc powders must be used.
Accordingly, an object of the present invention is to provide a negative electrode material for use in an alkaline cell capable of reducing the amount of a hydrogen gas which evolves from the cell. Another object of the present invention is to provide a cell which is low in the hydrogen gas evolution amount.
The present inventors have conducted intensive studies in order to solve the aforementioned problems of the prior art and have found that the hydrogen gas evolution amount can be remarkably reduced by employing a zinc powder all the particles of which are surface-coated with a homogeneous coating film consisting of bismuth, said zinc particles having a size within the range of 10 to 1000 xcexcm which is a commonly employed size range in connection with powders for use in the cell. The present inventors have also found the below-mentioned unexpected phenomenon which has enabled them to develop a novel method capable of preparing bismuth-coated zinc particles easily and at low cost. Namely, they have found that a substantial quantity of bismuth or bismuth oxide, which has heretofore been considered to be insoluble in an aqueous alkaline solution, dissolves therein and that the resultant dissolved bismuth in the aqueous alkaline solution performs displacement reaction with surfaces of zinc particles added therein to allow the surfaces of zinc particles to be coated with bismuth. Moreover, the present inventors have attained the present invention based on the finding that the uniform bismuth-coating films can be formed on the surfaces of zinc particles having a particle size range employable as a powder for use in a cell by adopting a step in which a bismuth powder and/or a bismuth oxide powder is dry mixed with a zinc powder and then the resultant mixture is immersed in an aqueous alkaline solution.
In accordance with one aspect of the present invention, there is provided a negative electrode material for use in an alkaline cell, said material comprising a powder consisting of bismuth-coated zinc particles which has been prepared by coating the surfaces of the zinc particles with bismuth and/or bismuth oxide.
In accordance with another aspect of the present invention, there is provided a negative electrode material as described above, in which said zinc powder is a zinc alloy powder.
In accordance with a further aspect of the present invention, there is provided an alkaline cell as described above, in which the zinc alloy powder contains at least one element selected from the group consisting of Al, Bi and In.
In accordance with a still further aspect of the present invention, there is provided a negative electrode material as described above, in which the bismuth-coated zinc powder consists of zinc particles having a particle size in the range of 10 to 1000 xcexcm.
In accordance with a still further aspect of the present invention, there is provided an alkaline cell characterized by having employed therein a negative electrode material as described above.
In accordance with a still further aspect of the present invention, there is provided a method of preparing a negative electrode material for use in an alkali cell, in which the bismuth-coated zinc powder can be prepared by a method comprising the steps of: dry mixing a zinc powder with a metallic bismuth powder previously; immersing the resultant mixed powder in an aqueous alkaline solution; and depositing by displacement the resultant dissolved bismuth on the surfaces of the zinc powder particles.
In accordance with another aspect of the present invention, there is provided a method of preparing a negative electrode material for use in an alkali cell, in which a bismuth-coated zinc powder can be prepared by a method comprising the steps of: dry mixing a zinc powder with a bismuth compound powder previously; immersing the resultant mixed powder in an aqueous alkaline solution; and depositing by displacement the resultant dissolved bismuth on the surfaces of the zinc powder particles.
In accordance with another aspect of the present invention, there is provided a method of preparing a negative electrode material as described above, in which the bismuth compound powder comprises a bismuth oxide powder.
In accordance with the other aspect of the present invention, there is provided a method of preparing the negative electrode material as described above, in which the zinc powder comprises a zinc alloy powder.